A) Field of the Invention
This invention relates to a liquid crystal display element.
B) Description of the Related Art
A vertical alignment type liquid crystal display element of which liquid crystal molecules in a liquid crystal layer align perpendicular to a substrate has a very good black level performance when no voltage is applied. Moreover, it has a very good viewing angle characteristic by introducing an optical compensator or optical compensators having a negative optical anisotropy with appropriate parameters on one side or both sides of a liquid crystal cell between upper and lower polarizers (for example, refer to Japanese Laid-Open Patent No. 2005-234254).
FIG. 17 is a schematic cross sectional view showing an example of a vertical alignment type liquid crystal display element according to the prior art.
A first substrate (upper substrate) 1 and a second substrate (lower substrate) 2 face each other, and a liquid crystal layer 3 is placed between them. The first substrate 1 consists of a transparent substrate 13, transparent electrodes (segment electrodes) 14 formed on one surface (inside surface facing the second substrate) of the transparent substrate 13, a vertical alignment film 15 coated on the transparent electrodes 14 and whose surface is treated by rubbing in a direction represented by an arrow 18, and a viewing angle compensator 12 and a polarizer 11 arranged on another surface (outside surface not facing the second substrate) of the transparent substrate 13. The second substrate 2, as same as the first substrate 1, consists of a transparent substrate 23, transparent electrodes (common electrodes) 24 formed on one surface (inside surface facing the first substrate) of the transparent substrate 23, a vertical alignment film 25 coated on the transparent electrodes 24 and whose surface is treated by rubbing in a direction represented by an arrow 28, and a viewing angle compensator 22 and a polarizer 21 arranged on another surface (outside surface not facing the first substrate) of the transparent substrate 23. The liquid crystal layer 3 includes liquid crystal molecules which align almost perpendicular to the surfaces of the substrates 1 and 2, and is given a pre-determined pretilt angle from a direction perpendicular to the substrate by the rubbing. Below the lower substrate 2, a back light 4 is placed.
The two upper and lower polarizers 11 and 12 are arranged to approximately crossed Nicols, and the absorption axis of one polarizer is arranged at about 45 degrees from an alignment direction of the liquid crystal molecules in the center of the thickness of the liquid crystal layer (hereinafter called the liquid crystal layer center molecules). The absorption axis is arranged at about 45 degrees to left and right or up and down direction of the liquid crystal display element. The viewing angle compensators 12 and 22 are made of a film having negative uniaxial optical anisotropy or negative biaxial optical anisotropy. In case of a film having negative biaxial optical anisotropy, its in-plane slow axis is preferably perpendicular to the absorption axis of the adjacent polarizer.
In case of the liquid crystal display element shown in FIG. 17, the rubbing directions of the upper and the lower substrates are set to the 12 o'clock direction (the direction represented by the arrow 18) and the 6 o'clock direction (the direction represented by the arrow 28) respectively to align them in an anti-parallel alignment. The liquid crystal material has a negative dielectric anisotropy, and the liquid crystal molecules are varied in the alignment configuration in parallel to the substrate surface in a bulk region of the liquid crystal layer 3 when a voltage is applied between the transparent electrodes 14 and 24 on the upper and the lower substrates.
FIG. 18 is a schematic plan view showing electrode patters of the transparent electrode (segment electrode) 14 and the transparent electrode (common electrode) 24 shown in FIG. 17. The same reference numbers as in FIG. 17 are added to the same components, and so explanations for the same components will be omitted.
In FIG. 18, the upper electrodes are the segment electrodes 14 having a strip (oblong) electrode shape extending to the 6-12 o'clock direction, and the lower electrodes are the common electrodes 24 having a strip (oblong) electrode shape extending to the perpendicular direction to the upper electrodes. Each rectangle region where the segment electrode 14 and the common electrode 24 cross each other composes one pixel.
With the above-described structure, it is possible to arrange an optimal viewing direction with the optimal display quality in the 6 o'clock direction of the liquid crystal display element. On the other hand, there is an anti-viewing direction in a direction rotating 180 degrees from the optimal viewing direction. The anti-viewing direction is a direction from which a light display part becomes very dark and display contrast decreases when observed at a certain polar angle from the direction to the liquid crystal display element. It is possible to obtain relatively good viewing angle properties from the left and right directions of the liquid crystal display element, which are the directions perpendicular to the optimal viewing direction and the anti-viewing direction.
Although there is no twisted alignment by rubbing on the upper/lower substrates in the liquid crystal layer 3, a twisted alignment can be generated by adding chiral dopant to the liquid crystal material. Moreover, it is not necessary to arrange the rubbing directions of the upper and the lower substrates in the anti-parallel alignment. The rubbing process may be performed only to one substrate, and the rubbing directions of the upper and the lower substrates may be twisted each other. In case that the rubbing directions are twisted each other, the alignment of the liquid crystal layer center molecules are aligned to up and down direction of the liquid crystal display element.
In case that the vertical alignment type liquid crystal display element according to the prior art shown in FIG. 17 is observed at a driving voltage which can obtain the maximum contrast, uniformed display is realized in a whole dot matrix display region when viewed from the optimal viewing direction and from the left and right directions of the element even if observation polar angles are changed. On the other hand, display uniformity is not sufficient when viewed from a range of directions rotated by 60 degrees clockwise and counterclockwise from the anti-viewing direction, and a display quality is considerably dropped especially when viewed from the anti-viewing direction.